1. Field of the Invention
The present invention is directed generally to quantum well infrared photodetector focal plane arrays (QWIP FPA's) and, more particularly, to QWIP FPAs that are capable of multicolor detection.
2. Background Information
Multicolor infrared detection capability has wide applicability to a number of different applications including aerospace, medical, surveying, mining, and agriculture. For example, multicolor infrared detection can provide useful information during geographical surveys. Detection in at least two infrared (IR) spectal bands could permit differentiation between man-made structures and natural landscape in a geographical survey. Additionally, in medical applications, multi-color detection would permit improved thermal imaging of the human body for diagnostic purposes.
Multicolor infrared detection has been conventionally performed using a wide-band IR detector and an associated rotating mechanical multicolor filter wheel. The wide-band IR detector detects a broad range of incident wavelengths. The rotating filter wheel selects the desired wavelength that is to be passed to the wide-band detector. An exemplary mechanical color filter wheel system is disclosed in U.S. Pat. No. 5,300,780. Mechanical color wheel systems, however, suffer from a number of deficiencies in multicolor detection. Such systems generally are slow and bulky, require large amounts of power for operation, and have a limited life span. Additionally, color wheel systems tend to have poor photon collection efficiency.
To alleviate some of the known deficiencies of mechanical color filter wheel systems, quantum well photodetectors have been constructed that permit the detection of more than one spectral band. Such detectors are described, for example, in U.S. Pat. Nos. 5,013,918, 5,198,659, and 5,384,469. These patents generally disclose the use of a plurality of quantum well sets within a single detector where the frequency response characteristics of each quantum well in the set is tailored by adjusting the well depths. The detectors in these patents are designed for use with a single bias voltage that is applied across all the quantum wells of the set. Thus, each quantum well that is “tuned” to a specific frequency band can not have its bias adjusted independent of any other “tuned” quantum well in the set. The output voltage responses for each of the “colors” in the detector will therefore be nonuniform for any given background photon flux. In addition, the detectors of these patents provide only a single detected output signal composed of multi-spectral frequency components. Extraction of different frequency bands from the single output therefore requires additional filtering.
Accordingly, there exists a need in the art for a quantum well photodetector that has multi-color capability but which can also provide independent bias adjustment for each color and which can independently and simultaneously process the outputs of each of the colors of the detector.